1. Field of the Invention
The present invention relates to a circuit termination method and a circuit terminating apparatus, and especially to the circuit termination method and the circuit terminating apparatus that are used in a metallic circuit and the like for high-speed communication.
2. Description of the Related Art
Various transmission systems have been studied for utilization with existing metallic circuits for high-speed data transmission. A metallic circuit tends to be subjected to unstable circuit conditions, and transmission quality may deteriorate, as compared with an optical cable.
Conventionally, when a circuit condition for a modem used on a common carrier leased line got worse, required communication quality under the poor circuit condition was generally achieved by using a fall back facility of the modem and thus lowering a clock rate to a terminal.
In the case of the modem, a clock signal line in a terminal interface is an independent line, and the modem adjusts an amount of data input and output according to width of a clock pulse.
Conversely, in the case of a high speed data transmission terminal that uses an existing metallic circuit between an office and a subscriber, the terminal is usually provided with a frame multiplex interface (for example, I-interface) with no independent clock signal line, therefore, the conventional technology cannot be used as it is.
When lowering a transmission speed due to deteriorated transmission quality, it is important to provide a service that corresponds to the class of the terminal and a level of reduction of the transmission speed such that a minimum transmission quality of the whole network is maintained.
As to the high speed data transmission terminal, there are various implementations, such as a TDM (Time Division Multiplex) terminal that physically multiplexes two or more channels, a frame relay terminal that transmits two or more logical channels over one physical interface, and an ATM (Asynchronous Transfer Mode) multiplexing apparatus, in addition to ones that offer one communication channel over one interface.
When changing the transmission speed of the variety of terminal apparatuses in response to deteriorated transmission quality, a circuit terminating apparatus has to control providing and suspending of service to each terminal.